The cellular industry which was until now unassumingly built on the basic premise of operating over licensed spectrum is considering the usage of operation on unlicensed band in order to meet the surging traffic demands. The unlicensed bands are typically dominated by Wi-Fi and other technologies. 3GPP has already decided to customize LTE standards in the LTE Release 13 for operation on the unlicensed bands and to exist harmoniously with Wi-Fi and other technologies. Unlicensed band is free to be used by any technology but is governed by few regulations (in most countries) like the requirement of “Listen Before Talk—LBT” which requires a transmitter to sense the channel for at least 20 us and if the channel is found to be free (not used by other devices) then it is allowed to transmit. Further the regulations allow for transmissions up to a maximum time limit and also provide means for giving fairness to the other devices/technologies.
In the legacy 3GPP LTE system, the concept of carrier aggregation is used in which multiple carriers can be allocated to a multi-carrier capable UE in order to boost the data rates, as depicted in FIG. 1. FIG. 1 is a schematic diagram 100 illustrating allocation of multiple carriers to the UE, according to the existing art. According to FIG. 1, one carrier is referred to as the Primary carrier 102 and the other carriers are referred to as the secondary carriers carrier1 104 and carrier2 106. The sub-frame boundaries on all the carriers are considered to be aligned. The scheduling can be self-carrier based or cross carrier based. In the self-carrier mode, the PDCCH for a secondary carrier is sent on the secondary carrier itself while in cross carrier mode, the resource allocation for all the secondary carriers is contained in the PDCCH that is sent on the primary carrier only.
FIG. 2 is a schematic block diagram 200 illustrating the processing of the receiving data packet and also preparing of the corresponding HARQ ACK/NACK, according to the prior art illustration. The HARQ operation is performed per carrier, wherein considering the DL scenario, the PDCCH can be sent either on the PCell (Cross Carrier Scheduling) or on the corresponding SCell (Self Scheduling). The HARQ acknowledgement is sent on the PUCCH which till Release 12 of LTE is carried on the PCell. The HARQ processes are defined per carrier and the DL HARQ operation is asynchronous in time which implies that the re-transmission (after receiving the negative acknowledgement NACK) can be sent by the eNB in any sub-frame after minimum time duration referred to as RTT (Re-Transmission Time). This aids in the flexibility of the eNB scheduler. The HARQ acknowledgement (ACK/NACK) is always sent after a pre-specified number of subframes with respect to the subframe in which data packet was received. In LTE, the pre-specified value is 3 subframes. This time accounts for the processing of the receiving data packet to determine if it is decoded correctly or not and also includes the time to prepare the corresponding HARQ ACK/NACK.
Further it is to be noted that the HARQ scheme in LTE is based on the incremental redundancy HARQ protocol in which a number of redundancy versions packets are created the original encoded data packet that is to be transmitted. A redundancy version packet is transmitted as the first transmission and if the UE sends a NACK then a next redundancy version packet is transmitted. The UE then attempts to decode the packet by combining both the redundancy version received at the first transmission and the redundancy version packet received at the first re-transmission. If the decoding is a success then the UE sends a ACK and the HARQ process completes else the UE sends a NACK and the eNB transmits another redundancy version packet to the UE. The UE then attempts to decode the packet by combining the first transmission, the first and the second re-transmission redundancy version packets received. Then it sends ACK/NACK subsequently. This process is repeated until the UE sends the ACK or until all the redundancy version packets are transmitted. In LTE 4 redundancy version (RV) packets are defined. The sequence of transmission is also pre-specified to be RV0, RV2, RV3 and RV1.
3GPP has started to work on utilizing the unlicensed bands for cellular communication. It has been agreed to add an unlicensed carrier in the legacy carrier aggregation framework of LTE. The unlicensed carrier is assumed to work in a license assisted manner. The standalone usage of unlicensed carrier is not conceived for the initial phases. This system is further referred to as LTE-U.